The use of liquid crystal polymers films as security devices has been reported in prior art. EP 0 435 029 discloses a data carrier with an optically variable security element that contains an oriented liquid crystal polymer. U.S. Pat. No. 5,678,863 (corresponding to GB 2,268,906) discloses a security marking for a document of value comprising a watermark coated with a cholesteric liquid crystal material producing optical effects which differ when viewed in transmitted and reflected light. The cholesteric liquid crystal material is for example a liquid crystal polymer.
However, the devices described in prior art have several drawbacks. Thus, the security element described in EP 0 435 029 is limited inasmuch that only the reflected colour of the liquid crystal polymer can be seen in the final element, since the liquid crystal layer is produced as part of a laminate structure with a solid black background. Also, the production of the device security element requires several steps, such as preparing the liquid crystal polymer film, transferring the liquid crystal polymer film from one substrate into another and then forming a laminate structure.
The security marking described in U.S. Pat. No. 5,678,863 implies a pre-treatment of the document of value to impart a watermark, or areas of differing optical density. The liquid crystal coating applied will appear differently in reflection and transmission and in addition will enhance the area of the watermark by appearing differently in areas of different optical density.
An aim of the present invention is to provide an optically variable marking, in particular for decorative and security applications, that does not have the drawbacks of the prior art devices, is easy to manufacture and can be applied to a broad variety of substrates, surfaces and objects.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
The inventors of the present invention have found that the above aims can be fulfilled by providing an optically variable marking comprising a semiopaque substrate with a first surface that is covered with an optically variable layer, and a second surface that is covered with metal layer a part of which is removed in the shape of a pattern. The optically variable layer is for example a liquid crystal polymer layer or a layer comprising pearlescent pigments dispersed in a transparent binder. When the marking is viewed from its metal layer side, the pattern is visible as a dark region in the metal layer. When the marking is viewed from its transflective layer side, in vertical rumination the optically variable layer appears as a uniform layer showing a reflection colour that varies with the viewing angle, whereas in transmission the pattern in the metal layer can be seen through the optically variable layer and the semiopaque substrate.
The optically variable marking according to the present invention overcomes the limitations of EP 0 435 029 by having both reflective and transmissive views possible. An advantage over the security markings described in U.S. Pat. No. 5,678,863 is that the document itself needs no pre-treatment but can include the marking as part of its structure. For example, the described marking can be added to the document for example by weaving it into the paper using a process known to one skilled in the art or by applying it as a transfer using a hot stamping foil process. In addition, in the present invention no visible markings are seen when the device is viewed in reflection, giving the document a hidden feature. Furthermore, the optically variable marking of the present invention is easy to manufacture, as will be shown below.
Another advantage of the optically variable marking of the present invention compared to prior art devices is that the optically variable film can be made ready for use in one roll to roll process involving coating, laminating, curing and rewinding. At this stage the product is a reflecting film. Later treatment of the film by exposure to laser energy can remove the metallised layer in defined regions. The design can therefore be changed without the necessity of changing the material of the document itself.
Definition of Terms
The term “optically variable” as used in this application means layers, coatings, devices, pigments and materials that are substantially transparent when viewed in transmission and, when viewed against a dark substrate, show a reflection colour that varies with the viewing angle. Typical examples of optically variable materials or pigments that are known in prior art are, for example, cholesteric liquid crystal coatings with planar alignment or platelet shaped liquid crystal pigments prepared from such coatings, as well as interference or pearl-lustre pigments, like for example flakes of mica, SiO2, Al2O3, TiO2 or glass that are coated with one or more layers of, e.g., titanium dioxide, iron oxide, titanium iron oxide or chrome oxide or combinations thereof.
The term ‘film’ as used in this application includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.
The term ‘marking’ includes films or layers covering the entire area of a substrate, as well as markings covering discrete regions of a substrate for example in the shape of a regular pattern or image.
The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal or mesogenic compound’ should denote materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal phase behaviour. Liquid crystal compounds with rod-shaped or board-shaped groups are also known in the art as ‘calamitic’ liquid crystals. Liquid crystal compounds with a disk-shaped group are also known in the art as ‘discotic’ liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerised.
For the sake of simplicity, the term ‘liquid crystal material’ is used hereinafter for both liquid crystal materials and mesogenic materials, and the term ‘mesogen’ is used for the mesogenic groups of the material.
The director means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axis (in case of discotic compounds) of the mesogens in a liquid crystal material.
The term ‘planar structure’ or ‘planar orientation’ refers to a layer or film of liquid crystal material wherein the director is substantially parallel to the plane of the film or layer.
The term ‘homeotropic structure’ or ‘homeotropic orientation’ refers to a layer or film of liquid crystal material wherein the director is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal.